rack and pinion.

[greencactus3]

has anyone tried to use a rack and pinion mechanism on their robot before? well, with a cylinder pushing the rack. not a motor spinning the pinion. that ive seen. also seen screw drives or w/e theyre called.
so, i would like to create a rack and pinion mech. few problems tho.
1. this is for the OCCRA competition. and in its rules it states NO precision tools allowed. ex) no cnc.
2. well, theirs lotsa rules, but mainily, unless we can hacksaw a flat gear rack from angle aluminum, we'll hafta use a chain as the rack.
3. no welding.
so, first q.
how to mount a piece of chain securely with no slippage to a piece of flat or angle aluminum without welding.. i was thinking zip ties, but i hope sumonell think of sumthing a bit more reliable.
next. how to keep the "rack" straight without falling off the sprocket. (remember, the "rack " is connected to a cylinder.)
and then the main question. is this gonna work?

so, anyone ever tried this idea of creating a rotary force with a cylinder before? we need a minimum of 140 degrees of motion. our cylinder we chose is a 2in stroke. so with a small sprocket, cant be too hard. except the chain may break? i dunno.

ps. no rotary cylinders in our kit...


well, we hope sumone can tip us on what would be difficult, improbable, easy "shortcuts" or even a picture of an example, it would be greatly appreciated.
main point.. we want help!!! aaand oh, one more problem. our first competition is coming up REAL soon!

[Max Lobovsky]

If you would be ok with using a cylinder with significantly the force that you actually need, you could wrap the chain part way around the sprocket with one end connected to the piston and the other end connected to a spring with a fixed mount. You would waste a lot of force (half the force?) tensioning the spring.

If you only need 140 degrees of motion, a better solution is probably to attach the cylinder to a spool of some sort with a cable. If i'm not mistaken, the maximum amount of force the cylinder produces will be applied through the whole 140 degrees as opposed to using a fixed linkage (like a cam).

[Adam Y.]

how to mount a piece of chain securely with no slippage to a piece of flat or angle aluminum without welding.. i was thinking zip ties, but i hope sumonell think of sumthing a bit more reliable.

There is a chain that has mounting points on it. From there you could bolt the chain to the piece of angle aluminum.

[greencactus3]

Originally Posted by Max Lobovsky

If you would be ok with using a cylinder with significantly the force that you actually need, you could wrap the chain part way around the sprocket with one end connected to the piston and the other end connected to a spring with a fixed mount. You would waste a lot of force (half the force?) tensioning the spring.

yes. i think thats a easy way. and then ill have to do sum math about the force of our cylinder. ill look into it. thanks!

If you only need 140 degrees of motion, a better solution is probably to attach the cylinder to a spool of some sort with a cable. If i'm not mistaken, the maximum amount of force the cylinder produces will be applied through the whole 140 degrees as opposed to using a fixed linkage (like a cam).

uhh. cant quite picture it. sorry. a simple paint sketch might help me. sorry im incapable.
and then i have to add, i want this compact.

There is a chain that has mounting points on it. From there you could bolt the chain to the piece of angle aluminum.

i doubt we are allowed any other kind of chain than the "normal" ones. sorry. OCCRA rules are pretty tight. thanks for the reply tho!

For this years FIRST robot (Daisy III) we actually used some 35 chain as a track to run our arm on. You can simply use screws or bolts (1 or 2 on each end) through the part of the chain that a gear tooth normall runs. Bolt it to some oak wood or alum. or the like and off you go! If the chain needs to be more taught, then use a router to put a recess in the material (wood or delran is nice) and then bolt it down.

Good luck with your project, Michael Greenley, Team 341

[Chris Fultz]

Try mounting th echain inside a piece of u shaped aluminum track. Something like what mounts to the wall for an adjustable shelf system. You can buy the track at most hardware / home stores. Cheap.

[greencactus3]

Originally Posted by Chris Fultz

Try mounting th echain inside a piece of u shaped aluminum track. Something like what mounts to the wall for an adjustable shelf system. You can buy the track at most hardware / home stores. Cheap.

again, i cant quite imagine what you are trying to tell me.. a quick paint sketch would be lovely.

[Ian Mackenzie]

How about something like this? Putting the chain around an idler (which could be as simple as a little bronze bushing on a bolt) avoids the need for elaborate tracks and ensures the chain stays in contact with the sprocket. It should be pretty easy to put a bolt or something through one link and attach it to the end of the piston with a bit of aluminum angle.

Piston and chain mechanism

EDIT: I don't know what's up with that thumbnail, but it doesn't look a whole lot like the actual image...

[Max Lobovsky]

And the same thing can be done with a cable and pulleys instead of chain and sprocket. The obvious advantages to cable are lighter overall weight, cheaper, and adjusting length is easier (use a turnbuckle). Are there any advantages to using chain?

[Justin Stiltner]

You could use the chain in the traditional sense, with 2 sprockets and the chain running around those like you would with a motor powered sprocket then attach the rod of the cylinder to the chain in the middle span between the sprockets. This way the cylinder powers the chain in both directions without using springs, ect. If you want a good example of what I mean go look at the curtains on your stage.... 2 pulleys, and your hand grabs one of the ropes, you can either pull down on it or push up on it (which actually pulls the rope up) this way your arm is powered in all directions of movement, which I have learned is a very good thing, gravity is nice, but with cables and such which can tangle when they become slack I prefer to power them in both directions.

[Max Lobovsky]

Originally Posted by Justin Stiltner

If you want a good example of what I mean go look at the curtains on your stage.... 2 pulleys, and your hand grabs one of the ropes, you can either pull down on it or push up on it (which actually pulls the rope up) this way your arm is powered in all directions of movement, which I have learned is a very good thing, gravity is nice, but with cables and such which can tangle when they become slack I prefer to power them in both directions.

If you complete a loop with the cable/rope, there is no problem with slack (rather, just about as much problem with slack as you would have with chain) So, again, I'm not sure why one would use chain in this situation.

[Al Skierkiewicz]

Green,
There are several good suggestions, I like Chris's chain in a "U" extrusion as a simple and effective approach. But a simple and lighter alternative is to wrap the chain around a sprocket (180 degree wrap) and terminate in a spring. The spring holds tension on the chain as the cylinder moves the sprocket. There is no need to have a bearing surface under the rack and no reason to hold the chain in alignment since that is automatic. This is similar to the loop approach discussed but doesn't add the weight of the second sprocket, bearing, and extra chain.

[Chris Fultz]

Originally Posted by greencactus3

again, i cant quite imagine what you are trying to tell me.. a quick paint sketch would be lovely.

Try this ms word picture. Hopefully it helps. Also, try a home store. Ask for small metal shelf hardware - slotted channel for the wall and metal shelf brackets. Should be easy to find.

Edit: go to www.lowes.com
Search for item 117881.